Packet communication system with QoS control function

ABSTRACT

A packet communication system of the present invention has first mode, second mode and third mode to apply to input packets. The first mode is a mode that decides priority of the packet by at least one of the address information and the application information, the second mode is a mode that decides priority of the packet by the DS value, the third mode is a mode that decides rewrite the DS value by at least one of the address information and the application information. A control unit of the packet communication system switches a mode to apply an input packet of the first mode, the second mode and the third mode based on the packet header information of the input packet.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention is related to a packet communication system withQoS control function, especially applicable to Diffserv.

[0003] 2. Description of Related Art

[0004] Increase of Internet users, follows a sharp increase in thetraffic (packets) that flows in the Internet. According to the packetcommunication mode used in the Internet, one line can carry packets frommany users. Therefore, the packet communication mode reduces cost per abandwidth. And strict management such as QoS (quality of service)control of every user, is not carried out in the Internet. This is alsoa factor of low cost of the Internet.

[0005] Telephone networks and company networks were constructed usingexclusive lines respectively. But the low cost of the Internet hascaused the movement of integrating telephone networks and companynetworks into the Internet for reduction of communication cost. For theintegration, it is preferable to provide QoS guarantees such as lowtransmission delay, low packet discard rate, and so on, because thosewere realized in conventional telephone networks and company networks.

[0006] To provide the QoS guarantees, a packet is transmitted based onthe priority of the packet that is decided by the agreement between aservice provider (SP) and the network user, such as a company, withdifferentiating the applications, such as telephone traffic, or itsrespective users.

[0007] Japanese Unexamined Patent Publication Disclosure 6(1994)-197128(related art (1)) shows a packet switching system that an output bufferfor CBR and an output buffer for VBR are installed in every each outputcircuit. The output priority of a packet accumulated to the buffer forCBR is higher than that of a packet accumulated to the buffer for VBR.

[0008] Generally, an ATM (Asynchronous Transfer Mode) switching systemsets a connection in advance according to an connection informationtable thereof. The connection information table also stores priorityinformation.

[0009] But a router does not have a connection information table becausea router, which is used in packet communication mode, does not set up aconnection in advance. Therefore, to provide QoS guarantees by a router,it is required a flow detecting means that detects priority informationof a packet based on information in the packet header. A router doespriority transfer of a packet based on the priority information detectedby the flow detecting means. In this specification a condition forpacket discrimination generated by information in the packet header, iscalled a flow condition. A series of traffic that meets the flowcondition is called a flow. And processing that decides whether an inputpacket meets the flow condition, and decides necessary information forQoS control, such as priority information, called flow detecting.

[0010] Japanese patent Laid-open print No. 6-232904 (related art 2)shows a priority control system in a router that decides relayprocessing priority from priority information and protocol informationof a received packet.

[0011] On the other hand, Diffserv (differentiated services) is statedin RFC 2475 of IETF (Internet engineering task force) (related art 3).Using FIG. 2, we explain related art 3. Company networks 221, 222, 223,and 224 are mutually connected by the DS domain 225. The DS domain 225executes the QoS control based on a policy such as TELNET ispreferentially processed. As a result, QoS that contracts in advancebetween the enterprise network users and an administrator of DS domain225, is provided. The DS domain 225 is composed of boundary node 226 andboundary node 227 that are positioned in the edge of the DS domain 225and interior node 228 that is positioned in the core of the DS domain225. Interior node 228 has much flow. And high-speed lines are connectedto interior node 228. Therefore, interior node 228 may not be able toperform QoS control with high speed. Diffserv is a solution to thatproblem. The Interior node has only limited function because the lode ofthe interior node is higher than that of the boundary node.

[0012] Suppose that a packet is transmitted from company network 221 tocompany network 224. When boundary node 226 receives a packet fromcompany network 221, flow detection means (It is called Classifier inRFC 2475) of boundary node 226 performs flow detecting using thesource/destination IP address, the source/destination port number, andthe protocol in the TCP/IP header as flow condition. And it decidespriority of the packet in the DS domain 225 and writes the priority intothe DS field of the packet header. Boundary node 227 and interior node228 with high load, performs flow detecting and QoS control with highspeed based on only DS field value.

SUMMARY OF THE INVENTION

[0013] In this specification, a network that Diffserv is applied iscalled a Diffserv network. At the time of shifting to a Diffservnetwork, there is less possibility of replacement of all existingrouters simultaneously because it is required to reduce the cost and therisk following the replacement to a minimum. Therefore, to shift to theDiffserv network smoothly, it is expected that the shift consist of twostages, that is “transition stage” and “practical use stage”.

TRANSITION STAGE

[0014] It is called “hot spot” that the point in the network that packetdiscard or increase of transmission delay happens. The router positionedhot spot will be replaced with a router with the powerful QOS controlselectively. To reduce a hot spot will improve communication quality.

[0015] The router in the transition stage is required a function thatperforms flow detecting using the source/destination IP address, thesource/destination port number, and the protocol in the TCP/IP header asflow condition, and decides the priority of the packet. In thisspecification, we call this function “Diffserv Mode 1”.

PRACTICAL USE STAGE

[0016] In the transition stage, when the replacement to a router withQoS control advances, QoS of the network will improve. When most ofrouters are replaced to a router with QoS control, the networkadministrator of the DS domain will start application of the Diffservnetwork. A router used as an interior node in this stage will berequired a function that judges priority information by the DS field. Inthis specification we call this function “Diffserv Mode 2”.

[0017] On the other hand, a router used as a boundary node in this stagewill be required followings. A router at the exit of the DS domain willbe required Diffserv FUNCTION 2. And a router at the entrance of the DSdomain 225 will be required a function that executes flow detectingusing the source/destination IP address, the source/destination portnumber, and the protocol in the TCP/IP header as flow condition, andjudges the priority of the packet and renewing the DS field inaccordance with the result of flow detecting. The function is called“Diffserv Mode 3” in this specification.

[0018] Therefore, to shift Diffserv network smooth, a router used as aninterior node will be required to support “Diffserv Mode 1” and“Diffserv Mode 2”, and switch the functions in accordance with thestage.

[0019] A router used as a boundary node will be required to support“Diffserv Mode 1”, “Diffserv Mode 2” and “Diffserv Mode 3”, and doswitching with the functions in accordance with the stage. In practicaluse stage, a router used as a boundary node will be required to support“Diffserv Mode 2” and “Diffserv Mode 3”, and do switching the functions.Furthermore, in practical use stage, the Diffserv function switchingwill be executed in accordance with position in the DS domain 225 suchas the edge node or the core node. For example, boundary node A226 willhave to apply Diffserv Mode 3 to an input packet from company network221 and Diffserv Mode 2 to an input packet from interior node 228. Onthe other hand, interior node 228 will have to apply Diffserv Mode 3 toall input packets. Moreover, an interior node executes “Diffserv Mode 2”at high speed because high-speed lines are connected to an interiornode.

[0020] However, related art 3 does not teach such viewpoints at all.

[0021] The object of present invention is to present a router that cando switching with Diffserv Mode 1 and Diffserv Mode 2.

[0022] Another object of present invention is to present a router thatthat can do switching with Diffserv Mode 1, Diffserv Mode 2 and DiffservMode 3.

[0023] Another object aim of present invention is to present a routerthat can do switching with Diffserv Mode 2 and Diffserv Mode 3.

[0024] Another object of present invention is to present a router thatcan do Diffserv mode switching in accordance with the position in the DSdomain 225 and/or DS domain 225 architecture.

[0025] Another object of present invention is to present a router thatcan execute “Diffserv Mode 2” at high-speed.

[0026] To achieve the object, a packet communication system of thepresent invention has at least two modes to apply an input packet offirst mode, second mode and third mode, the first mode being a mode thatdecides priority of the packet by at least one of the addressinformation and the application information, the second mode being amode that decides priority of the packet by the DS value, the third modebeing a mode that decides rewrite the DS value by at least one of theaddress information and the application information. A control unit ofthe packet communication system switches a mode to apply an input packetof the modes based on the packet header information of the input packet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a block diagram of a router of a first embodiment of thepresent invention;

[0028]FIG. 2 is a schematic view for explaining Diffserv network;

[0029]FIG. 3 is an example of a packet format used by a network of thepresent invention;

[0030]FIG. 4 is an example of an internal packet format used by a routerof the present invention;

[0031]FIG. 5 is IP address format;

[0032]FIG. 6 is a format of an entry table of a first embodiment of thepresent invention, showing a condition wherein a Diffserv mode is set upin input line units;

[0033]FIG. 7 is a flowchart of a first embodiment of the presentinvention, showing a condition that a Diffserv mode is set up in inputline units;

[0034]FIG. 8 is a block diagram of a flow detecting unit of anembodiment of the present invention, showing a condition that a Diffservmode is set up in input line units;

[0035]FIG. 9 is an example of a format of priority table;

[0036]FIG. 10 is an example of a format of Diffserv mode table;

[0037]FIG. 11 is a format of an entry table of a second embodiment ofthe present invention, showing a condition that a Diffserv mode is setup in entry units;

[0038]FIG. 12 is a block diagram of controller of a second embodiment ofthe present invention, showing a condition that a Diffserv mode is setup in entry units; and

[0039]FIG. 13 is a flowchart of a second embodiment of the presentinvention, showing a condition that a Diffserv mode is set up in entryunits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT EMBODIMENT 1

[0040]FIG. 1 is a bock diagram of a router of a first embodiment of thepresent invention. Router 100 has header processing unit 110, packetinput/output unit 120 for transferring a packet, and processor 130. Theheader processing unit 110 has ARP processing unit 113 for performingARP (Address Resolution Protocol) processing, routing processing unit111 for performing routing processing and flow detecting unit 112 fordetecting flow. The packet input/output unit 120 has output FIFO(FirstIn First Out) buffer distribution circuit 121, line interfaces 122-i(i=1, . . . ,N) and lines 123-i (i=l, . . . ,N). Control terminal 140and network management equipment 150 are connected to the processor 130.

[0041]FIG. 3 is an example of a packet format used by a network of thepresent invention. The packet format provides packet header unit 310 anddata unit 320. The header unit 310 provides source MAC (Media AccessControl) address (SMAC) field 300, destination MAC address (DMAC) field301, source IP (Internet Protocol) address (SIP) field 302, destinationIP address (DIP) field 303, source port (SPORT) field 304, destinationport (DPORT) field 305, and DS (Differentiated Service) field 306. SMACshows the physical address (hardware address) of the last router thattransferred the packet, and DMAC shows the physical address (hardwareaddress) of the next router where the packet is transferred. SIP showsthe IP address of the source terminal that transfers the packet, and DIPshows the IP address of the destination terminal where the packet istransferred. SPORT and DPORT show the protocol, that is, the hostapplication program. DS shows the priority of the packet in DS domain225. The data unit 320 provides user data field 321. The header unit 310also comprises information on upper protocol over IP, which may behandled like the information mentioned above. Moreover, although FIG. 3shows the packet format that the protocol of transport layer is TCP(Transmission Control Protocol) or UDP (User Datagram Protocol) and theprotocol of network layer is IP, a protocol of transport layer ornetwork layer may be another protocol. For example, a protocol ofnetwork layer may be IPX (Internetwork Packet Exchange).

[0042]FIG. 4 is an example of an internal packet format used by a router100 of the present invention. The packet format is that interior headerunit 330 is added to the packet format shown in FIG. 3. The interiorheader unit 330 is comprised of input line number 307, output linenumber 308, and priority information 309. The input line number 307shows the number of line where the packet was inputted, and the outputline number 308 shows the number of line where the packet is outputted.The priority information 309 is used in performing priority transfer.Another format analogous to the format shown in FIG. 4 may be used.

[0043] We referee FIG. 1 again. When a packet is inputted from inputline 123-i, receiver circuit 124-i transforms the packet into theinternal packet format shown in FIG. 4, wherein the receiver circuit124-i provides the input line number i to the input line number 307 ofthe internal packet format. After that, the receiver circuit 124-itransmits the interior packet to input FIFO buffer 126-i. At this time,the output line number 308 and the priority information 309 of theinternal packet have no meanings yet. The input FIFO buffer 126-i storespackets, and transmits them to output FIFO buffer distribution circuit121 in order of arrival. The output FIFO buffer distribution circuit 121stores the packets into buffer 128 and transmits header information 11,which is composed of header unit 310 and internal header unit 330, tothe header processing unit 110.

[0044] The routing processing unit 111 retrieves the routing table inthe unit, which is not shown in FIG. 1, according to the DIP 303 of theheader information 11. Due to the retrieval, the routing processing unit111 decides the output line information 12 to transmit the packet to thesub-net that the DIP belongs to and the IP address of the next router,that is NIP (Next Hop IP Address) information 14. The processor 130provides and manages the routing table. Japanese Unexamined PatentPublication Disclosure 10(1998)-222535) discloses retrieval of a routingtable. Routing processing unit 111 transmits the output line information12 to the output FIFO buffer distribution circuit 121 and transmits NIPinformation 14 to ARP processing unit 113. When the output FIFO bufferdistribution circuit 121 receives the output line information 12, theoutput FIFO buffer distribution circuit 121 provides the output lineinformation 12 to the output line number 308 of the packet stored in thebuffer 128.

[0045] When the ARP processing unit 113 receives the NIP information 14,the ARP processing unit 113 decides the DMAC information 15corresponding to the NIP information 14 and outputs the DMAC information15 to the output FIFO buffer distribution circuit 121. When the outputFIFO buffer distribution circuit 121 receives the DMAC information 15,the output FIFO buffer distribution circuit 121 provides the DMACinformation 15 to the DMAC 301 of the packet stored in the buffer 128.

[0046] On the other hand, flow detecting unit 112 retrieves entry table850, decides priority information 13 for priority transmission, DSrewriting effective information 16 for indicating execution/notexecution of rewriting DS and rewrite DS information 17 and output themto the output FIFO buffer distribution circuit 121.

[0047] When the output FIFO buffer distribution circuit 121 receives thepriority information 13, the output FIFO buffer distribution circuit 121provides the priority information 13 to the priority information 309 ofthe packet stored in the buffer 128. And when the output FIFO bufferdistribution circuit 121 receives the DS rewriting effective information16 and the DS information 17, the output FIFO buffer distributioncircuit 121 rewrites the DS 306 to the DS information 17 if the DSrewriting effective information 16 indicates the effective, and does notrewrite the DS 306 if not so. After that, the output FIFO bufferdistribution circuit 121 decides the line interface 122-k (k=1, . . . N)based on the output line number 308 and output FIFO buffer 127-kj(j=1,2) on the line interface 122-i based on the priority information309. In this embodiment, the output FIFO buffer 127-k1, 127-k2 are forhigh priority and low priority respectively. The output FIFO buffer127-kj stores the packet. Transmission circuit 125-k controls thereadout from the output FIFO buffer 127-kj. The readout control may becomplete priority, weighted round robin, and so on. In the completepriority, if packets is stored in the output FIFO buffer 127-k1 for highpriority, the packets are read out in order of arrival. If no packet,packets stored in the output FIFO buffer 127-k2 for low priority areread out in order of arrival. On the other hand, in the weighted roundrobin, packets stored in FIFO buffer 127-k1 and packets stored in FIFObuffer 127-k2 are read out based on a predetermined ratio. The controlin the transmission circuit 125-k is set up by the network managementdevice 150 or control terminal 140. The transmission circuit 125-kcancels the internal header unit 330, provides the MAC address allocatedto line 123-k to the SMAC 301 and transmits the packet to the line123-k.

[0048] Next, we explain detailed operation of the flow detecting unit112. FIG. 8 is a block diagram of the flow detecting unit 112 of anembodiment of the present invention. The flow detecting unit 112 hasresult decision unit 810, coincidence decision unit 820, entry readoutunit 830, controller 840 and entry table 850. The controller 840 hasDiffserv mode table 841 that is used in setting up mode 1, 2, and 3 ininput line units and Diffserv mode decision unit that decides theDiffserv mode based on the input line number.

[0049]FIG. 10 is an example of a format of Diffserv mode table 841. Mode1 is a preferable mode to be realized the transition stage. In Mode 1,the Diffserv Mode 1 is applied to the flow detecting unit 112. That is,the flow detecting unit 112 performs flow detecting using thesource/destination IP address, the source/destination port number, andthe protocol in the TCP/IP header as flow condition, and decides thepriority of the packet. Mode 2 and Mode 3 are preferable modes to berealized the practical use stage. In Mode 2, Diffserv Mode 2 is appliedto the flow detecting unit 112. That is, the flow detecting unit 112judges priority information of a packet by the DS field of the packet.In Mode 3, Diffserv Mode 3 is applied to the flow detecting unit 112.That is, the flow detecting unit 112 performs flow detecting using thesource/destination IP address, the source/destination port number, andthe protocol in the TCP/IP header as flow condition, and judges thepriority of the packet and renewing the DS field in accordance with theresult of flow detecting. Administrator of DS domain 225 can build theDiffserv mode table 841 using the control terminal 140 or the networkmanagement device 150 through the processor 130.

[0050]FIG. 6 shows a format of an entry table 850. The entry table 850has H entries 630. Each of the entries flow condition 631 and QoScontrol information 632. The QoS control information 632 is composed ofpriority information 611 for a priority transfer and rewrite DSinformation 612. The flow condition 631 is composed of a condition todistinguish the source or the destination of the packet and a conditionto distinguish the protocol.

[0051] The flow condition to distinguish the source or the destinationof the packet is SIP upper limit 601, SIP lower limit 602, DIP upperlimit 603, DIP lower limit 604, IP effective bit 621 to indicate thatthe upper and lower limit of SIP and DIP are effective, input linenumber 607 and input line number effective bit 623 to indicate thatinput line number 607 is effective. The boundary node 226 and theboundary node 227 shown in FIG. 2 can understand which transferred thepacket of the company networks from 221 through 224 by the input linenumber. A subnet, which means a domain of IP network divided by a subnetmask, can be designate by only an entry 630 if the upper limit and lowerlimit of SIP or DIP is set up.

[0052]FIG. 5 shows IP address format. IP address 540 is composed ofnetwork address 541 and host address 542. A subnet is distinguished bythe network address 541 a terminal in the subnet by the host address542. As the high-order bits of the IP address 540 designates a networkaddress, the terminals in the network have continuous IP addressesrespectively. Therefore, a range of IP addresses defined by an upperlimit and an lower limit can designate the terminals.

[0053] The flow condition to distinguish the protocol is SPORT 605 toindicate a source port, DPORT 606 to indicate a destination port andport effective bit 622 to indicate that the SPORT 605 and the DPORT 606are effective. If the flow detecting is performed with IP address, portnumber and input line number, then “Effective” is set to the IPeffective bit 621, the port effective bit 622 and the input line numbereffective bit 623 respectively; otherwise, then “Invalid” is setrespectively.

[0054]FIG. 7 shows a flowchart for explaining the processing of the flowdetecting unit 112. The processing of the flow detecting unit 112 isroughly divided into four parts. Those are detecting starting processing700, entry readout processing 730, condition coincidence deciding 720and result deciding 710. The entry readout processing 730, the conditioncoincidence deciding 720 and the result deciding 710 are performed byentry readout unit 830, coincidence decision unit 820 and resultdecision unit 810 respectively, which are shown in FIG. 8.

[0055] We explain the processing of the flow detecting step by stepreferring FIG. 7 and FIG. 8, which shows a block diagram of a flowdetecting unit 112. In the detecting starting processing 700, when theheader information 11 of the packet is transmitted to the headerprocessing unit 110, the flow detecting unit 112 stores the input linenumber 307, SIP 302, DIP 303, SPORT 304, DPORT 305 and DS 306 intomemory for line No. of packet 826-2, memory for SIP of packet 822-2,memory for DIP of packet 823-2, memory for SPORT of packet 824-2, memoryfor DPORT of packet 825-2 in the coincidence decision unit 820 andmemory for DS in result decision unit 810 respectively (Step 710).Diffserv mode decision unit (no illustration) in controller 840 decidesthat it is the Diffserv mode that the corresponding value of Diffservmode table 841 to the input line number of the memory for line No. ofpacket (Step 704). In Mode 1 or Mode 3, the Diffserv mode decision unittransmits a start signal to the entry readout unit 830 (noillustration).

[0056] The processing in Mode 1 or Mode 3 is as follows. In the entryreadout processing 730, when receiving the start signal, the entryreadout unit 830 sets the number “M” of entry No. counter for “1” toread out the first entry 630-1 of the entry table 850 (Step 731). Then,entry table address generator 832 generates an address of the entrytable 850 based on the value of M, reads out the entry 630. Moreover,the entry table address generator 832 transmits the SIP upper limit601-1 and the SIP lower limit 602-1 of the entry to memory for SIP ofentry 822-3, the DIP upper limit 603-1 and the DIP lower limit 604-1 ofthe entry to memory for DIP of entry 823-3, SPORT 605-1 of entry tomemory for SPORT of entry 824-3, DPORT 606-1 of entry to memory forDPORT of entry 825-3 and the IP effective bit 621-1, the port effectivebit 622-1 and the input line number effective bit 623-1 to memory foreffective bit 827. And the entry table address generator 832 transmitsthe priority information 611 -1 and the rewrite DS information 612-1 tomemory for priority 813 and memory for rewrite DS 816 in the resultdecision respectively (Step 732). Then, The value of M is incremented byone to read out the second entry 630-2 of the entry table 850 at nextentry readout processing (Step 733).

[0057] In the condition coincidence decision processing 720, thecoincidence decision 820 decides whether the input packet agree with theflow conditions stored in the memory for SIP of entry 822-3, the memoryfor DIP of entry 823-3, the memory for SPORT of entry 825-3, the memoryfor DPORT of entry 826-3 and the memory for line No. of entry 826-3.

[0058] SIP compare circuit 822-1 compares SIP upper limit 601 and SIPlower limit 602 stored in the memory for SIP of entry 823-3 with SIPstored in the memory for SIP of entry 822-3. If the SIP satisfies thecondition such that

SIP lower limit 601 ≦SIP≦SIP upper limit 602,

[0059] or IP effective bit 621 is “Invalid”, then SIP compare circuit822-1 decides to be coincidence (step 721-1). DIP compare circuit 823-1performs a processing like SIP compare circuit 822-1 to DIP (step721-2). If SPORT stored in memory for SPORT of packet 823-2 with SPORT605 stored in memory for SPORT of entry 823-3 or PORT effective bit 622is “Invalid”, then SPORT compare circuit 824-1 decides to be coincidence(step 721-3). DPORT compare circuit 825-1 performs a processing likeSPORT compare circuit 824-1 to DPORT (step 721-4). If input line numberstored in memory for line No. of packet 826-2 with input line No. 607stored in memory for line No. of entry 826-3 or input lien numbereffective bit 623 is

EMBODIMENT 2

[0060] In Embodiment 1, Diffserv Mode is set in every input line unitsas shown in FIG. 6. To change Diffserv mode more flexibly, it ispreferable to set Diffserv Mode in entry units, that is, in flow units.Hereinafter, we mainly explain the deference between embodiment 1 andembodiment 2.

[0061]FIG. 11 shows a format of entry table 1150 which replaces entrytable 850 shown in FIG. 6 and FIG. 8. As shown FIG. 11, Diffserv Mode1100 is added to each of entries 630. An administrator of DS domain 225sets up Diffserv Mode 1100 by control terminal 140. The setting may beperformed by network management device 150.

[0062]FIG. 12 shows controller 1240 which replaces controller 840 inFIG. 8. Controller 1240 has memory for Diffserv Mode 1241 in stead ofDiffserv Mode table 841.

[0063]FIG. 13 shows a flowchart for explaining the processing of theflow detecting unit 112 of embodiment 2. In step 1332 in entry readoutprocessing 1330, processing of storing Diffserv Mode 1100 in memory forDiffserv Mode 1241 is added to step 732 in FIG. 7. In step 1334,Diffserv Mode is decided by the value stored in memory for Diffserv Mode1241.

[0064] As above-mentioned, the present invention provides a router thatis useful in shifting to Diffserv network.

[0065] Although the present invention has been described in connectionwith a preferred embodiment thereof, many other variations andmodifications will now become apparent to those skilled in the art.

What is claimed is:
 1. Packet communication system for transmitting apacket, the packet having header information including addressinformation, application information and DS filed value, comprising: aplurality of input lines; a plurality of output lines; a switching unitconnected to said input lines and said output lines, the switching unitoutputting the packet from an input line of said input lines to anoutput lines of said output lines; and a control unit for switching amode to apply to the packet of first mode, second mode, and third modebased on the header information, the first mode being a mode thatdecides priority of the packet by at least one of the addressinformation and the application information, the second mode being amode that decides priority of the packet by the DS value, the third modebeing a mode that decides rewrite the DS value by at least one of theaddress information and the application information.
 2. Packetcommunication system according to claim 1, wherein said control unitcomprises a table indicating which mode to apply to each of input linesof the first mode, the second mode and the third mode, whereby saidcontrol unit decides the mode to apply to the packet.
 3. Packetcommunication system according to claim 1, wherein the packet is a IP(Internet Protocol) packet and the address information is a source IPaddress.
 4. Packet communication system according to claim 1, whereinthe packet is a IP (Internet Protocol) packet and the applicationinformation is a TCP (Transmission Control Protocol) port number.